The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present technology.
Alloy road wheels with high magnesium or aluminum content are not uncommon on specialty and racing vehicles. The use of such alloy wheels in less expensive passenger vehicles has, however, been limited to a few production sports cars. By way of example, galvanic corrosion is a design consideration in high magnesium content alloy wheels when mated to steel or cast iron wheel hub and brake components. Frequently, these components may spend much of their service life in damp or wet conditions, unfortunately often with road salts, which accelerates the galvanic corrosion reactions. Various coatings have been applied to light metal workpieces and substrates (e.g., comprising magnesium or aluminum), such as alloy wheels, for increasing corrosion protection, but they have had many drawbacks. For example, workpieces having only thick oxide layers formed thereon have been used, but are often brittle and prone to cracking. Workpieces having powder coating materials directly applied to oxide layers have shown poor adhesion. Workpieces having chemical passivation techniques in combination with an oxide layer have been used, but have had poor chipping resistance. Still further, workpieces simply having an electrocoating layer provided on an oxide layer have also been used, but may yield a product with poor scratch corrosion and poor thermal shock resistance. In yet other alternatives, wheels may be provided as two-component assemblies having inner and outer portions, with the inner portion galvanically isolating the outer portion from the steel or cast iron wheel hub and brake components. However, such two component assemblies may not always be desirable.
Accordingly, there remains a need for improved surface treatments for increased corrosion resistance of light metals and alloys susceptible to corrosion.